Which of the following combinations describe the triple bond in an alkyne group? (A) The triple bond consists of two 𝜎 bonds and one 𝜋 bond. (B) The triple bond consists of one 𝜎 bond and two polar bonds. (C) The triple bond consists of two 𝜋 bonds and one polar bond. (D) The triple bond consists of one 𝜎 bond and two 𝜋 bonds. Or (E) the triple bond consists of two 𝜎 bonds and one dative bond.
Alkynes are a series of organic compounds, all with the same general formula C𝑛H2𝑛−2, where 𝑛 is a nonfractional number such as one, two, three, and so on. Alkyne molecules contain a carbon-carbon triple bond functional group called an alkyne group. R and R dash can be an alkyl group, for example the CH3 group which is a methyl group, or C2CH3, an ethyl group. Or R and R dash can be an aryl group, for example, a benzyl group. Or they can be a hydrogen atom.
The question asks about the triple bond. We need to identify what types of bonds constitute this triple bond. To do that, we need to know what happens to each of these carbon atoms during bonding. In the outer shell of each carbon atom, there are two electrons in the 2s subshell. And these electrons are paired. And there are two electrons in the 2p subshell. And these electrons are unpaired. When a carbon atom becomes excited or gains energy, one of the electrons can be promoted to the 2p subshell. Hybridization, or mixing of two of these orbitals, can occur, resulting in four orbitals, two of which are still two 2p orbitals and the other two are new orbitals called sp hybrid orbitals.
The four electrons arrange themselves into these orbitals as follows. Each sp hybrid orbital gains one electron, and the two 2p orbitals each have one single electron. Each 2p orbital has a typical dumbbell shape, whereas the sp hybrid orbitals also have a dumbbell shape, but one of the lobes is large and one of the lobes is smaller. The two carbon atoms at a triple alkyne bond each have two 2p orbitals, one in the 𝑦-plane and one in 𝑧-plane. Each orbital contains one unpaired electron, which are these unpaired electrons here. Each of the carbon atoms also has two sp hybrid orbitals. Here is one for the first carbon and a second for the first carbon. And each of these sp orbitals contains one unpaired electron, these two over here. Let’s do the same for the second carbon atom.
Now, let’s have a look at how these two sp hybridized carbon atoms form a triple bond. The first bond between the two carbon atoms forms from the head-on overlap of an sp hybrid orbital from one carbon atom with an sp hybrid orbital from another carbon atom. This is a 𝜎 bond. The second and third bond which forms between the two carbon atoms forms as a result of the overlap between the p orbitals from each carbon atom in the 𝑦-plane and the overlap of the p orbitals from each carbon atom in the 𝑧-plane. These two bonds are called 𝜋 bonds and may be drawn or represented something like this, with a 𝜋 bond looking a bit like a broken doughnut in the 𝑦-plane and the other in the 𝑧-plane.
Don’t forget, each carbon atom also has an sp hybrid orbital available for bonding here and here. And so carbon could form a single bond over here and over here. Finally, which of the following combinations describe the triple bond in an alkyne group? The answer is (D). The triple bond consists of one 𝜎 bond and two 𝜋 bonds.